As is known, plants for the rapid drying of cut tobacco use a process gas with temperature higher than 200° C., to rapidly evaporate the water contained in the particles of cut tobacco.
More in detail, the majority of these drying plants are provided with a long straight drying duct arranged vertically, traditionally called “drying column”, inside which a more or less constant flow of process gas is made to flow upwards with a given quantity of suspended cut tobacco particles per unit of volume, so as to cause rapid drying of the tobacco particles as they ascend the duct; and a big cyclone filter, which is connected to the upper end of the drying duct so as to receive at the inlet the process gas with the particles of tobacco in suspension, and is structured so as to cause precipitation of the particles of cut tobacco onto the bottom of the cyclone filter, discharging the process gas from an outlet usually located on the top of the cyclone filter.
More specifically, in this type of drying plant, the process gas is fed to the inlet of the drying duct by a hot gas generator, which is usually structured so as to suck the process gas from the cyclone filter, heat it to a temperature higher than 200° C., and lastly pump it back to the inlet of the vertical drying duct.
The particles of cut tobacco are fed into the flow of process gas immediately upstream of the vertical drying duct, by means of a star valve which is usually located at the lower end of the drying duct, and is structured so as to be able to introduce into the drying duct a more or less constant flow of particles of cut tobacco, without causing discharge of the process gas from the drying duct.
The bottom of the cyclone filter is provided with a second star valve which allows extraction of the particles of cut tobacco from the cyclone filter, without discharging the process gas from the cyclone filter.
Since the quantity of water extracted from the tobacco particles is a function of the residence time of the particles in contact with the high-temperature process gas, and the height of the “drying column” cannot be greater than the maximum height of the warehouse in which the drying plant is installed, in recent years some manufacturers of plants for rapid drying of cut tobacco have begun to use drying ducts provided with a short rectilinear starting section which extends parallel to the ground, and to locate the star valve along the horizontal rectilinear section of the drying duct.
In this way, the particles of cut tobacco remain in contact with the process gas for the entire length of the vertical rectilinear section and also for all or part of the horizontal rectilinear section of the drying duct, with all the resulting advantages.
Unfortunately, while allowing a significant increase in the working length of the drying duct, the presence of the 90° bend on the duct causes anomalous distribution of the particles of cut tobacco in the flow of process gas which ascends the vertical section of the drying duct, at least partly thwarting the advantages due to the increased working length of the drying duct.
Experimental tests, in fact, have shown that, downstream of the bend, the particles of cut tobacco tend to stratify close to the wall of the duct which is arranged outside the bend, locally increasing the density of particles per unit of volume beyond the maximum limits admissible for optimal drying of the particles of cut tobacco.
In addition, at the bend, the particles of cut tobacco tend to knock violently against the wall of the duct depositing on the surface tar-based oily residues which, over time, stratify on the surface of the wall, forming scale which further worsens the distribution of the particles of cut tobacco along the vertical section of the drying duct, and furthermore progressively reduces the free section of the drying duct, compromising correct operation of the drying plant in the long term.